Mass spectrometry continues to expand in application and importance. Much of this activity arises from new ionization sources that, for example, expand existing capabilities and/or allow new analytical techniques.
Mass spectrometers can be separated into two categories: those that possess atmospheric pressure sampling inlets, and those that possess vacuum sampling interfaces. Instruments operating at atmospheric pressure are typically equipped with an electrospray ionization source, which generates ions from solutions at atmospheric pressure and is commonly coupled to liquid chromatography. Instruments possessing vacuum sampling interfaces are often equipped with matrix-assisted laser desorption/ionization (MALDI) sources. Since these ionization techniques are complementary in many ways, laboratories are normally outfitted with at least one of each. Because mass spectrometers are typically dedicated to one technique or the other, and since these instruments are expensive both to purchase and maintain, operating costs of a facility capable of both techniques can be high.
Matrix assisted laser desorption ionization (MALDI) can provide an ion source for the analysis of biologically important molecules. By use of a pulsed laser for one-step desorption and ionization, the technique shows application under both reduced pressure and atmospheric pressure conditions. But problems can remain, particularly in the lower Dalton range where high background can persist.
Corona discharges are widely used as electron emitters for primary ion formation in atmospheric pressure chemical ionization (APCI) sources (Dzidic I., Carroll, R. N., Stillwell, R. N., Horning, E. C. (1991) Anal Chem. 48:1763), providing efficient ionization of gas phase neutral molecules (Bruins, A. P. (1991) Mass Spec. Rev. 10: 53). Typically, these neutral molecules are delivered into the corona discharge region as gas or liquid chromatograph effluents (Harrison, A. G. (1992) Chemical Ionization Mass Spectrometry, CRC Press:Boca Raton, Fla., pp. 53).